Water-absorbent resins have been widely used in the field of hygienic materials, such as disposable diapers, sanitary napkins, and incontinence pads, by utilizing the characteristics of the water-absorbent resins that large amounts of aqueous liquids, for example, body fluids such as human urine, blood, and sweat, are rapidly absorbed and that the liquids once absorbed are not released even under loads.
For example, crosslinked products of partially neutralized polymers of acrylic acid are preferred as water-absorbent resins because they have many advantages, including the followings: they have better water-absorption performance; their raw materials such as acrylic acid has easy industrial availability, and therefore they can be produced with stable quality and low cost; they show no shortcomings such as in which decomposition is likely to occur; and they are safer products.
These water-absorbent resins are required to have better water-absorption performance. Specifically, it is required to have appropriate liquid-absorbent capacity, water-absorption rate, liquid suction force, water-absorption capacity under a load, gel strength, and so on. In addition to such water-absorption performance, from the viewpoint of being used in absorbent articles requiring cleanliness, such as diapers and sanitary articles, it is required to be less discolored and to be scarcely discolored with passage of time. That is, the water-absorbent resins have problems that they are easily yellowed or browned by external factors, such as heat and humidity, during storage. In particular, in the field of hygienic materials, if a water-absorbent resin in an absorbent article, such as a disposable diaper or a sanitary napkin, is discolored, the commercial value of the article significantly decreases. The water-absorbent resin is therefore required to be scarcely discolored even after storage under a severe environment.
Examples of the technique for preparing a water-absorbent resin having better water-absorption performance include a method of producing a water-absorbent resin by mixing a water-absorbent resin containing carboxyl groups and a plurality of crosslinking agents having different solubility parameters and heating the mixture (see Patent Document 1), a method of producing water-absorbent resin particles by polymerization using a water-soluble azo radical polymerization initiator in the presence of a multivalent glycidyl compound as an internal-crosslinking agent (see Patent Document 2), and a method of producing a water-absorbent resin by performing a polymerization reaction in the presence of a diamine compound or its salt and performing a crosslinking reaction by adding a crosslinking agent after the polymerization (see Patent Document 3). Examples of the water-absorbent resin having a discoloration-preventing effect include a highly water-absorbent polymer composition composed of a highly water-absorbent polymer and an organophosphate compound or its salt (see Patent Document 4), a water-absorbing agent composition including an acid water-swelling crosslinked polymer, a basic water-swelling crosslinked polymer, and a discoloration-preventing agent and/or an antioxidant and/or a boron compound (see Patent Document 5), and a water-absorbing agent composition including a water-absorbent resin, an organic carboxylic acid, and/or its salt (see Patent Document 6).    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H6-184320    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2006-176570    Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2008-133396    Patent Document 4: Japanese Unexamined Patent Application, Publication No. H5-86251    Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2000-230129    Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2000-327926